Method for minimizing the temperature coefficient of resistance of passive resistors in an integrated circuit process flow

ABSTRACT

An integrated circuit containing a resistor and the resistor per se. The circuit includes a substrate ( 2 ), a semiconductor resistor ( 3 ) on the substrate and a layer of electrically insulating material ( 5 ) disposed over the substrate and the semiconductor resistor having at least one contact ( 11, 13, 15 ) extending therethrough to the semiconductor resistor, the contact having an electrical path therein extending to and forming an interface with an end portion of the semiconductor resistor. The semiconductor resistor has a semiconductor resistor body, preferably of doped polysilicon, having one of a positive or negative temperature coefficient of resistance and a resistor head. The resistor head consists essentially of the electrical path which is metal interconnect, the contacts and then interface to and from the resistor body and in contact with the resistor body, the resistor head having the other of a positive or negative temperature coefficient of resistance.

This application is a divisional of Ser. No. 09/207,344, filed Dec. 8,1998, now U.S. Pat. No. 6,211,769 and claims priority under 35 U.S.C.119(e)(1) based on provisional application Ser. No.60/068,467 filed Dec.22, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a system for fabricating passive resistors inintegrated circuits that display minimal change in resistance over awide range of operating temperatures.

2. Brief Description of the Prior Art

Integrated circuits are generally fabricated with polysilicon resistorsthat are formed on the semiconductor substrate. Such resistors generallyhave a resistor body, generally formed of doped polysilicon andgenerally having metallic leads coupled to opposing ends of the resistorbody, generally through contacts in field oxide. The contacts areconnected to metal interconnect. The resistor body can be formedconcurrently with polysilicon transistor gate electrodes, such resistorbody generally doped and generally resting on the field oxide.

Integrated circuits that require passive resistors often have tighttolerances on the resistance value of these resistors. However, theseprior art semiconductor resistors are subject to variations inresistance value. Sources of variation in the resistance value of theseresistors include process fluctuations that result in physical changesto the resistor properties such as physical dimension or resistivity andchanges in temperature. The sources of change in temperature can beeither external to the resistor itself or internal due to theself-heating effects associated with power dissipation. As the resistortemperature changes, the value of resistance of the resistor alsochanges.

The general prior art method utilized for minimizing the resistancealteration effects due to the temperature coefficient of resistance(TCR) of a semiconducting resistor (a resistor formed of semiconductormaterial) is to increase the doping concentration in the resistor bodyto a sufficiently high level such that the TCR of the resistor body isat a minimum. Then the resistors are built with dimensions that make theresistor head resistance a small percentage of the resistor bodyresistance. As a result, the resistor head TCR has little effect on theoverall resistor temperature characteristics.

To reach sufficiently low TCR conditions in the resistor body alone, theimpurity or doping concentration must be very high, about 3×10²⁰atoms/cm³ for polysilicon resistors. Extra processing steps are oftenrequired to reach this level of impurity concentration. These steps addcost to the production of the circuit and limit the range of resistancevalues obtainable in that the sheet resistance (R_(s)) is low (about 70to about 100 ohms/square). The simple expression for resistance isR=R_(s)(L/W), where L is length and W is width of the resistor body.This equation shows that to achieve the desired value of R when R_(s) islow, W must be minimized (which increases the head component ofresistance and increases process variability) and/or L must beincreased, the latter increasing capacitance and area consumed on thechip. The increase in length is also detrimental at high frequency wherethese resistors are sometimes used.

SUMMARY OF THE INVENTION

In accordance with the present invention, the above describedinadequacies of prior art resistors are minimized.

The change in resistance with temperature of a semiconducting materialcan be modeled by a numerical fit that takes the form:

R(T)/R(T ₀)=1+TCR 1×(T−T ₀)+TCR 2×(T−T ₀)²

where the resistance at temperature T is R(T), T₀ is the initial orreference temperature and TCR1 and TCR2 are the fitting coefficients forthe resistor body and the two resistor heads which make connection atopposite ends of the resistor body respectively. The above equationapplies to the body or the head separately. In other words, the aboveequation can be used for either the body or for the head of the resistorindividually. Then the coefficients TCR1, TCR2 which are the numericalfitting coefficients are found by fitting the equation to the data. Itcan be thought of as fitting the expression y=1+ax+bx² to some datawhere a and b are the fitting coefficients. For every case (level ofdoping concentration) that has been observed for polysilicon resistors,the second fitting coefficient TCR2 is several orders of magnitude lowerthan TCR1, so TCR2 has almost no effect on the equation over thetemperature range of interest, this being from about −55° C. to about140° C. This equation represents the best fit to the resistance datataken over the above range of temperatures. The temperature coefficientsof resistance TCR1 and TCR2 for the resistor body can be either negativefor low to mid levels of doping concentration or positive for very highlevels of doping concentration. The same statement applies to the headTCR1 and TCR2, but the head TCR1 and TCR2 generally do not change fromnegative to positive at the same doping concentration as the body.

When a semiconducting material is used for the resistor body, electricalcontacts are made to the resistor body in a region known as the resistorhead. Typically, the electrical path to the resistor body is madethrough metal leads and contacts and possibly a metal or metalliccompound in contact with the resistor body. For the present discussion,the combination of all of these components is considered to be theresistor head. The total resistance of the resistor structure can bewritten as R=R_(b)+2×R_(h), where R_(b) is the resistance of theresistor body and R_(h) is the resistance of each resistor head. BothR_(b) and R_(h) will have temperature characteristics as described bythe above equation in R(T)/R(T₀). The temperature coefficients of theresistor heads, which include the interface resistance (or contactresistance) between the resistor head material and the resistor body,can be different from the temperature coefficients of resistance of theresistor body. Metals and metallic compounds typically have positivecoefficients of resistance. The head-to-resistor body interfaces mayhave either positive or negative temperature coefficients of resistance,depending upon the doping concentration in the body.

When the resistor body is built with an overall negative coefficient ofresistance, the above equation in R(T)/R(T₀) when applied to R_(b)becomes less than 1 for temperatures greater than T₀ and the value ofR_(b) decreases with increasing temperature. When the resistor head isbuilt with an overall positive coefficient of resistance, the value ofR_(h) increases with increasing temperature or the R(T)/R(T₀) equationbecomes greater than 1 when applied to R_(h). With proper design of thephysical dimensions of the resistor heads and resistor body, themagnitude of the increase in resistance of the resistor heads can offsetthe magnitude of the decrease in resistance of the resistor body,resulting in very low overall change in resistance for the entireresistor structure.

The above described method of reducing changes in resistance due totemperature allows precision passive resistor structures to be builtwith materials that are already available in the process flow. No extraprocessing steps are generally required unless the polysilicon resistoris part of a self-aligned silicide process flow in which case oneadditional process step is required. The silicide must be blocked fromthe resistor body, usually by a patterned oxide or nitride. Extraprocessing can be provided to change the resistivity of the resistorbody or resistor heads, however the above solution provides the desiredresult, whether or not extra processing is utilized.

The procedure in accordance with the present invention generally doesnot require additional processing steps as required in the prior art toobtain the high doping levels since lower impurity concentrations in thesemiconductor which do not require the additional processing aregenerally adequate. In addition, the method in accordance with thepresent invention can take advantage of the positive TCR associated withmetal leads and contacts that are commonly used in the manufacture ofintegrated circuits. If only the resistor body TCR is minimized, thenthe methods used to electrically connect the resistor to the rest of thecircuit will only result in an increase in resistance with temperature.This change in resistance will be significant for resistors built withlow values of resistance. The resistor design described herein can beused to minimize the effects of changing temperatures on the change inresistance of the overall resistor structure.

A semiconductor resistor is fabricated in accordance with the presentinvention by providing a semiconductor resistor body, preferably ofdoped polysilicon with the doping level depending upon whether theresistor is to display a positive or negative TCR. The resistor isformed as a part of an integrated circuit and rests on the upper surfaceof or over a substrate. The resistivity also changes with dopingconcentration. Resistivity generally decreases with increasing dopingconcentration. Since the sheet resistance (R_(s)) is the resistivity (ρ)divided by the film thickness (t) or R_(s)=ρ/t, the sheet resistancevalue also changes with any change in the doping concentration chosen.Therefore, to provide a target value of resistance R, the resistorlength and/or width can be adjusted accordingly since R=R_(s) (L/W). Inpractice, generally the full equation for R, which includes the headcomponents, is used, this being R=R_(b)+2 R_(h)=R_(s) (L/W)+2R_(h).Typically, the resistor rests on field oxide, so it will be covered bythe material used to insulate metal from the substrate/polysilicon. Apair of contacts are formed through the electrical insulator at oppositeends of the resistor body. Each contact contains an electricallyconductive material, preferably a refractory metal such as, for example,titanium in a thin layer with most of the contact filled with eithertungsten or aluminum. This forms a refractory metal silicide interfacewith the resistor body or polysilicon doped with, for example,phosphorus to a level to provide a thermal coefficient of resistancewithin the contact sufficient to offset the thermal coefficient ofresistance of the resistor body. A metal interconnect connects theconductor in the contacts to other components in standard manner.

There are many ways to achieve opposite signs for the body and head TCR.One example is to dope the polysilicon resistor body with a 1×10²⁰atoms/cm³ concentration of arsenic and a 2×10²⁰ atoms/cm³ concentrationof phosphorus. The body TCR1 has been demonstrated to be positive forphosphorus concentrations of about 2.4×10²⁰ atoms/cm³ and higher, andthe head TCR1 has been demonstrated to be positive for phosphorousconcentrations of about 1.6×10²⁰ atoms/cm³ and higher, a concentrationof 2×10²⁰ atoms/cm³ therefore providing a positive head TCR1 with anegative body TCR1 (TCR2 is being neglected because its effect isminimal in the temperature range of interest which is from about −55° C.to about 140° C.).

Phosphorus alone or boron alone can be used. The polysilicon body TCR1is positive for boron doping concentrations of 1.6×10²⁰ atoms/cm³ andgreater while the head TCR1 remains negative for all levels of dopingconcentration that have been observed, these levels being from 9×10¹⁹atoms/cm³ to 3.2×10²⁰ atoms/cm³. All of the doping concentrations listedabove change for different process flows. The polysilicon grain sizechanges, depending upon the deposition scheme used, film thickness, andthermal cycles in the process flow. The amount of activated dopant willchange as a function of the amount and type of thermal processing used.Both polysilicon grain size and dopant activation alter the TCRcharacteristics, thus requiring different doping concentrations withchanges in these factors. It follows that actual dopant values aresomewhat empirical and must be adjusted to the conditions.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic diagram of a semiconductor resistor inaccordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the FIGURE, there is shown a semiconductor resistor body 3which is formed as a part of an integrated circuit and rests on theupper surface of a field oxide 1 which rests over a substrate 2. Theresistor body 3 is formed of polysilicon and doped with phosphorus to alevel of 2×10²⁰ atoms/cm³ and with arsenic to a level of 1×10²⁰atoms/cm³ to provide a negative coefficient of resistance to theresistor body. A layer of insulating dielectric 5 rests over theresistor body 3 and the field oxide 1. A pair of contacts 11, 13 extendthrough the insulating dielectric at opposite ends of the resistor body3, each contact containing a titanium barrier layer with the aperturescontaining the contacts then being filled with tungsten or aluminum witha positive coefficient of resistance within the contacts. This forms aninterface of titanium silicide 17 with the resistor body 3 sufficient tooffset the negative coefficient of resistance of the resistor body. Thetitanium silicide 17 is formed during the standard SAlicide(self-aligned silicide) process. The titanium silicide is prevented fromforming on the resistor body by depositing an insulating material on thebody prior to SAlicide formation or by using the sidewall nitride. Ametal interconnect 15 connects the tungsten or aluminum 11, 13 in thecontacts to other components in standard manner.

The process used to prevent the titanium silicide from forming on theresistor body is as follows:

1) an insulator is deposited which may be an oxide or may use thesidewall nitride.

2) the insulator is patterned so that it covers only the resistor body.

3) the insulator is etched away from non-patterned areas. Thus, when thestandard titanium silicide process is used, it forms only on theresistor heads. The above steps are not required where a self-alignedsilicide process is not used.

Though the invention has been described with respect to a specificpreferred embodiment thereof, many variations and modifications willimmediately become apparent to those skilled in the art. It is thereforethe intention that the appended claims be interpreted as broadly aspossible in view of the prior art to include all such variations andmodifications.

What is claimed is:
 1. A method for minimizing the temperaturecoefficient of resistance of a semiconductor resistor as a part of aprocess of fabricating a semiconductor integrated circuit whichcomprises the steps of: (a) providing a partially fabricated integratedcircuit having a first dielectric surface region; (b) forming asemiconductor resistor body on said surface region, said resistor bodyhaving one of a positive or negative temperature coefficient ofresistance; (c) forming a mask over said semiconductor resistor body;(d) forming an aperture through said mask at at least one end region ofsaid resistor body extending to said end region of said resistor body;(e) forming a resistor head extending through said aperture, saidresistor head consisting essentially of an electrical path and interfaceto and from the resistor body and in contact with the resistor body,said resistor head having the other of a positive or negativetemperature coefficient of resistance which is substantially equal tothe temperature coefficient of resistance of said resistor body tooffset temperature changes of resistance of said resistor body; and (f)completing fabrication of said semiconductor integrated circuit.
 2. Themethod of claim 1 wherein said resistor body is doped polysilicon. 3.The method of claim 1 wherein said step of forming said resistor headcomprises the step of forming a refractory silicide at opposing ends ofsaid resistor body during formation of refractory silicide at otherlocations on said integrated circuit.
 4. The method of claim 2 whereinsaid step of forming said resistor head comprises the step of forming arefractory silicide at opposing ends of said resistor body duringformation of refractory silicide at other locations on said integratedcircuit.
 5. The method of claim 1 wherein said refractory metal istitanium.
 6. The method of claim 2 wherein said refractory metal istitanium.
 7. The method of claim 3 wherein said refractory metal istitanium.
 8. The method of claim 6 wherein said refractory metal istitanium.
 9. The method of claim 1 wherein said step of completingfabrication of said semiconductor integrated circuit comprises the stepof forming an interconnect with said resistor head disposed over saidfirst dielectric surface.
 10. The method of claim 8 wherein said step ofcompleting fabrication of said semiconductor integrated circuitcomprises the step of forming an interconnect with said resistor headdisposed over said first dielectric surface.